Pesticides are frequently used in agriculture to help keep pests, such as weeds and insects, under control. Herbicides are a subclass of pesticides and are used to kill unwanted plants. Selective herbicides target specific types of plants and so can be used on crops without harming them. For example, the herbicide atrazine is commonly used on farms that grow corn and sugar cane; it controls broadleaf and grassy weeds but does not damage the corn or sugar cane crops. Atrazine is very effective and relatively inexpensive, so it is one of the most widely used herbicides in the world.

However, the use of atrazine is controversial because of the potential health effects of atrazine exposure. Atrazine has become one of the most common contaminants in water supplies. Water runoff washes it from the soil of crops where it's been applied, into streams, rivers, lakes, and groundwater. Exposure to atrazine in drinking water can cause birth defects, low birth weight, and other reproductive issues. Animal studies have also shown damage to the liver, kidneys, and heart. Atrazine has been banned in the European Union. In the United States, atrazine is a restricted-use pesticide, meaning that it is not available for purchase or use by the general public; only trained and certified persons can apply atrazine, and there are also restrictions on when, where, and how it can be used.

In the U.S., the level of atrazine in water supplies is monitored for safety. The Environmental Protection Agency (EPA) has set the maximum contaminant level (MCL) for atrazine in drinking water to be .003 mg/L, or 3 parts per billion (ppb). This level was determined by evaluating scientific studies and assessing the potential health risks; as a precaution, the MCL is set at least 300 to 1,000 times more protective than the level at which harmful effects were observed in studies. EPA requires water suppliers to regularly test water samples, and if levels are consistently above the MCL, the supplier must notify the public and remove the contaminant so that it is below the MCL. Atrazine can be removed from water using granular activated charcoal. However, there is debate about whether these regulations are strong enough to protect people from atrazine. Because the MCL is an annual average, occasional peaks of higher concentrations (such as seasonal peaks) may not trigger a violation—so the public would not be notified that they were being exposed to higher concentrations at those times. Furthermore, some studies indicate that even low exposures below the MCL may be harmful to human health.

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